docs: def _and_previous(self):
          """Return a 4-tuple that will usually be (current branch, current
          revision - 1, current branch, current revision), unless
          current revision - 1 is before the start of the current
          branch, in which case the first element will be the parent
          branch.
  
          """
          branch = self.db.branch
          rev = self.db.rev
          (parent, parent_rev) = self.db.sql('parparrev', branch).fetchone()
          before_branch = parent if parent_rev == rev else branch
          return (before_branch, rev-1, branch, rev)